The present invention relates to a substrate on which a photocatalytic film having titanium oxide as a primary component is coated and a method for producing the same.
Substrates (articles) on whose surface a photocatalytic film is coated are widely used in, for example, windowpanes of buildings requiring fouling resistance, antibacterial property, and the like, indicator panels of electronic displaying instruments, casings of portable equipment, sanitary installations, frames of medical facilities, apparatus in the field of biotechnology such as DNA analysis and the like. The substrates on which a photocatalytic film containing titanium oxide as a primary component is coated are widely used in order to provide fouling resistance, antibacterial property, deodorizing function and the like to the such substrate surfaces.
Such photocatalytic films are coated by sputtering processes, and it is known that crystalline titanium dioxide films are preferable as such films in order to increase the photocatalytic activity of the photocatalytic films and make the fouling resistance performance better. However, in order to coat on a substrate a titanium dioxide film having good crystallinity, there is a problem of requiring heating the substrate to be coated at a temperature of 300xc2x0 C. or more (Japanese Patent Laid-Open No. 10-1 52396).
In addition, there are known photocatalytic films prepared by irradiating a metallic titanium target with electron beams in an oxidizing atmosphere to coat an amorphous titania (non-crystalline titanium oxide) film on a glass plate, followed by calcination it at a temperature of 400 to 500xc2x0 C. or more to form rutile-typed titania crystal (Japanese Patent Laid-Open No. 10-146251) and processes for photocatalytic films of crystalline titanium dioxide prepared by coating a metallic titanium or titanium oxide film on a glass plate by sputtering, followed by calcination it (Japanese Patent Laid-Open Nos. 10-278165 and 10-146251).
However, coating a photocatalytic film on a substrate heated at high temperatures by a sputtering process that are carried out in an atmosphere at reduced pressure causes releasing impurity gases such as moisture from the inner wall of the heated decompressing container (film-forming chamber container), and therefore has technical problems of being difficult in making reduced-pressure atmosphere good purity. In other words, in order to make reduced-pressure atmosphere good purity necessary to obtain a good photocatalytic activity, a vacuum exhausting system in a big way is required to sputtered-film-forming equipment, causing problems of increasing the cost of coating a photocatalytic film.
In addition, methods of crystallizing the above-mentioned amorphous titanium oxide and metallic titanium films in the air by heating and calcination have problems of poor productivity because of requiring calcination for a sufficient period of time at high temperatures (heat treatment) and expensive calcination equipment and elongating the heating cycle. Furthermore, there are problems of not obtaining readily photocatalytic films with good reproductivity.
A subject of the present invention is to solve the above-mentioned problems. A first purpose of the present invention is to provide a substrate on which a photocatalytic film having high photocatalytic activity is coated. A second purpose of the present invention is to provide a method capable of producing a substrate with such photocatalytic film without the need of heating at a high temperature in the film-coating step.
The present invention have been achieved by finding that when a titanium oxide film coated on the surface of a substrate by a sputtering process which is amorphous or at a state where the crystallinity has not sufficiently developed is heated and crystallized with controlling the oxygen-defect state of the film, it can be formed to a photocatalytic film containing titanium oxide as a primary component and having good photocatalytic activity.
One embodiment of the invention is a substrate with photocatalytic film comprising a substrate and a photocatalytic film coated on the substrate, wherein said photocatalytic film is coated on said substrate by sputtering a target containing titanium in an atmosphere at reduced pressure, contains titanium oxide as a primary component, and has a sheet resistance of 109 to 1013 xcexa9/xe2x96xa1.
The photocatalytic film having a titanium oxide film as a primary component according to the present invention is a crystalline titanium dioxide film containing anatase-typed crystal and characterized by a controlled oxygen-defect state of the film. In the case that a titanium dioxide film contains an almost stoichiometric amount of titanium and oxygen, its sheet resistance is at a level of 1015 xcexa9/xe2x96xa1 or more. By contrast, the photocatalytic film according to the present invention is formed to an oxygen-defect state, whereby the sheet resistance of the film can be controlled in the range of 109 to 1013 xcexa9/xe2x96xa1.
Although the reason is not clear why the degree of the photocatalytic activity of a titanium oxide film depends on the oxygen-defect state of the titanium dioxide film, it is likely that the oxygen deficiency can be contributed to the energy level of the crystalline titanium oxide film.
The relationship between the photocatalytic activity and the sheet resistance of the titanium dioxide film indicates that when the sheet resistance is more than 1013 xcexa9/xe2x96xa1 and the amount of oxygen deficiency is reduced, the photocatalytic activity is decreased. By contrast, when the sheet resistance is less than 109 xcexa9/xe2x96xa1, that is, when the oxygen deficiency is present at a more amount than that indicated by 109 xcexa9/xe2x96xa1 sheet resistance (the oxygen in the titanium oxide film is reduced to a smaller amount than that corresponding to the stoichiometric amount of titanium dioxide), the photocatalytic activity is reduced, if anything. The titanium oxide photocatalytic film according to the present invention has a sheet resistance controlled within the range of 109 to 1013 xcexa9/xe2x96xa1, and therefore has high photocatalytic activity.
The photocatalytic film according to the present invention is coated on a substrate by a sputtering process. The film is coated by employing metallic titanium or a metal having metallic titanium as a primary component as a target, and sputtering it in reduced pressure atmosphere comprising a mixed gas of an inert gas such as argon and oxygen or oxygen gas alone. The controlling of the oxygen deficiency of the titanium oxide film can be achieved by controlling the total pressure of the atmosphere gas and the partial pressure of oxygen during sputtering the target.
Also, the photocatalytic film according to the present invention can be coated by employing, as a target, titanium dioxide that has the stoichiometric composition or is at a state where the oxygen is somewhat more defect than the stoichiometric amount (sometimes referred to as titanium suboxide), and sputtering it in reduced pressure atmosphere comprising a mixed gas of an inert gas such as argon and oxygen or oxygen gas alone. The controlling of the oxygen deficiency in the titanium oxide film can be achieved by controlling the total pressure of the atmosphere gas and the partial pressure of oxygen during sputtering the target.
The sheet resistance of the photocatalytic film may be controlled so as to be within 109 to 1013 xcexa9/xe2x96xa1 by heat treatment after coating on the substrate.
In the present invention, the sheet resistance can be controlled such that the titanium dioxide film immediately after coating has the resistance range described above, and preferably is controlled in combination with the resistance controlling during coating and the resistance controlling by heat treatment after coating. This can result in the formation of a photocatalytic film having a higher surface hardness.
The heat treatment is carried out such that the contract angle of pure water on the surface of the photocatalytic film is 65xc2x0 or less. The degree of the photocatalytic activity and practical performance are affected by the hydrophilicity of the film surface. It is preferable that the heat treatment is carried out such that the sheet resistance of the photocatalytic film is controlled in the above-mentioned range and the hydrophilicity of the photocatalytic film is reduced. From this viewpoint, it is preferably that the contact angle of pure water on the surface of the photocatalytic film is 65xc2x0 or less. In the present invention, the contact angle of pure water refers to a contact angle under the condition where the films are allowed to stand in a dark room for two weeks after the exposure of UV irradiation (the center wave-length of 360 nm, 3 mW/cm2) for 30 min.
The photocatalytic film may contain, as a by-component, any one of niobium oxide, aluminum oxide, iron oxide and nickel oxide. Incorporating these metals into the crystal lattice of titanium oxide can change the energy level of the crystal lattice, thereby resulting in the formation of a film having higher photocatalytic activity.
A niobium oxide film, aluminum oxide film, iron oxide film or nickel oxide film may be interposed as an underlying film between the substrate and the photocatalytic film. For example, in the case that a niobium oxide film is the underlying film, a photocatalytic film obtained by laminating and coating a niobium oxide film and a titanium oxide film on the substrate and subsequently heat treating has the niobium diffused from the interface between the titanium oxide and niobium oxide films into the titanium oxide film, thereby resulting in higher photocatalytic activity. A niobium oxide film is preferable among these oxide films.
Substrates to be employed in the present invention are not limited to specified materials and shapes. Any material can be used which does not deteriorate during coating a film by sputtering, and on optionally heating the substrate and on the heat treatment after coating. For example, substrates of glass, ceramic, resin or metal can be used. In particular, for glass plate containing alkali components such as soda lime silicate glass plate, the underlying film can prevent the decrease in the photocatalytic activity by diffusing the alkali components of the glass into the titanium oxide photocatalytic film.
The invention includes a method for producing a substrate with photocatalytic film, comprising the steps of coating a photocatalytic film containing titanium oxide as a primary component on a substrate by sputtering a target containing titanium in an atmosphere at reduced pressure, and subjecting the photocatalytic film after said coating to heat treatment.
As a sputtering target used for sputtering in the present invention, sintered molded articles of fine powder of metallic titanium or titanium dioxide can be utilized. In the case of employing metallic titanium as the target, it is possible to increase the photocatalytic activity to a higher extent by incorporating a small amount of niobium, iron, aluminum, nickel or the like and changing the energy level of the resulting titanium dioxide photocatalytic film. In the case of employing sintered articles of titanium oxide as the target, it is possible to use sintered articles of fine powder of titanium dioxide, sintered molded articles of fine powder of titanium suboxide (having a somewhat smaller O/Ti ratio than that of titanium dioxide).
An oxygen atmosphere, a mixed gas atmosphere of oxygen and an inert gas such as argon and the like can be employed as the atmosphere at reduce pressure during coating the film. This is intended for the crystallization of the film coated on the substrate and the increase in the crystallinity of the film, and results in the controlling of the oxygen-defect state, as expressed by the sheet resistance, in the range of 109 to 1013 xcexa9/xe2x96xa1.
In the case that the sheet resistance of the coated film is more than 1013 xcexa9/xe2x96xa1, the heat treatment is carried out in an inert or reducing atmosphere.
For the films having a sheet resistance after coating of more than 1013 xcexa9/xe2x96xa1, the increase in the photocatalytic activity can be achieved by increasing the oxygen deficiency in the films. To this purpose, the atmosphere during the heat treatment is an inert or reducing atmosphere. The sheet resistance of the photocatalytic film is decreased to a value of 109 xcexa9/xe2x96xa1 or more.
The inert atmosphere is exemplified by a nitrogen-gas atmosphere, and an inert-gas atmosphere such as argon, or a vacuumed space where the pressure is reduced to a sufficient degree. An atmosphere containing hydrogen gas can be employed as the reducing atmosphere.
It is likely that the increase of the oxygen deficiency in the film are effected by drawing the oxygen having unstable or weak linkage with titanium from the inside of the film to the outside. In order to increase the photocatalytic activity, it is preferable that the sheet resistance value of the film is decreased to 109 xcexa9/xe2x96xa1 or more, and more preferable to 1010 xcexa9/xe2x96xa1 or more.
In the case that the sheet [surface] resistance of the photocatalytic film coated on by a sputtering process is less than 109 xcexa9/xe2x96xa1, the heat treatment is carried out under an oxidizing atmosphere.
The sheet resistance of the film of less than 109 xcexa9/xe2x96xa1 indicates that the film is rich in oxygen deficiency therein and has a small photocatalytic activity. In this case, the photocatalytic activity can be increased by decreasing the amount of the oxygen deficiency in the film, that is, by increasing a sheet resistance value corresponding to the amount of the oxygen deficiency. The oxidizing atmosphere employed can be an atmosphere containing oxygen or ozone, and the heat treatment in the air is most preferable from an economical viewpoint. The sheet resistance value of the photocatalytic film is increased to 1013 xcexa9/xe2x96xa1 or less.
It is preferable that the sheet [surface] resistance value of the film is increased to 109 xcexa9/xe2x96xa1 or more, and more preferable to 1010 xcexa9/xe2x96xa1 or more, in order that the oxygen deficiency in the film is disappeared and the photocatalytic activity is increased.
The heating temperature in the heat treatment step is from 200 to 350xc2x0 C. It is preferable that the heating temperature is from 200 to 300xc2x0 C.
It is possible to increase the degradation ratio of triolein of the photocatalytic film and the hydrophilicity of the film surface.
The sputtering target may contain[s] any one of niobium, aluminum, iron and nickel as a by-component. It is possible to contain more than one by-component.
It is preferable that in the case that a target to be used is a metallic target, these metals are contained at 0.1 to 3% by weight of the metallic titanium. The photocatalytic film obtained by sputtering such a metallic target in an oxygen-containing atmosphere has the oxygen-defect state that can be controlled by heat treatment in a similar way to as in the single composition of titanium oxide.
The method may further comprise the step of coating any one of niobium oxide film, aluminum oxide film, iron oxide film and nickel oxide film on the substrate by sputtering a target containing any one of niobium, aluminum, iron and nickel in an atmosphere at reduced pressure, before coating the photocatalytic film containing titanium oxide as a primary component.
For example, in the case of employing a niobium oxide film, a laminate of the niobium oxide and titanium oxide films laminated on the substrate has the niobium diffused into the titanium oxide film by the heat treatment, resulting in affecting the energy level of the photocatalytic film, and thus improving the photocatalytic activity. It is preferable that the substrate temperature is less than 200xc2x0 C. during coating the photocatalytic film on the substrate by sputtering.